1. Field of the Invention
This invention relates to drill bits for material such as wood and plastic. More particularly, the invention relates to a three-bladed spade-type drill bit having a conical countersink cutting point.
2. Description of the Prior Art
In drilling cylindrical holes in relatively soft materials such as wood or plastic, one of the more popular drill bits is the spade bit similar to that depicted in U.S. Pat. No. 3,997,279 issued to Porter. This type of bit is particularly useful in drilling reasonably accurate holes of a relatively large diameter.
The spade drill bit of the prior art has several disadvantages. Primarily the bit leaves rough edges around the entrance opening and significant chipping of the surface around the exit opening. Another disadvantage is that the conventional spade drill cannot be used to enlarge an existing hole. Further, since the spade drill bit of the prior art has only two cutting blades on opposite sides of the shaft, it is somewhat unstable requiring very careful control when in use.
The conventional countersink drill is a multi-bladed drill bit forming a conical point which is used for making limited depth impressions as around the entrance of a smaller diameter screw hole for recessing the screw head. the countersink drill may be used on pre-existing holes, is quite stable, and leaves smooth edges around the entrance hole.
The instant invention overcomes the disadvantages of the spade drill while retaining its advantages, and combines the features of the countersink drill. The drill of the instant invention makes cylindrical holes in material of various thicknesses with little or no chipping or splintering on the surface around the entrance or exit openings, is stable thereby requiring only moderate control, is self-centering, and may be used to enlarge existing holes. Further, the instant invention makes a cylindrical hole with a much smoother inner surface than provided by the conventional spade drill. In addition, because of the wide space between blades, maximum chip clearance is achieved.